1. Field of Invention
This invention pertains generally to anaerobic digesters and, more particularly, to a system and process that significantly extends the efficiency, control, and applicability of anaerobic digesters to all of the many and variously different liquefied bio-waste products over a wide variety of conditions and concentrations.
2. Introduction and Related Art
Designers of wastewater facilities have always been concerned with energy costs. Historically, however, rather large engineering xe2x80x9chousesxe2x80x9d have designed wastewater treatment plants (WWTPs) with hydraulic loads of millions of gallons per day.
Designing these plants involved large structures, the design responsibility for each was assigned to a person or group that specialized in that particular function. A process engineer who was forced by the dictates of Federal, State, and local agencies to aim the overall design performance almost entirely with respect to the effluent requirements of these entities determined the make up of the process itself. Although energy considerations have in recent years received some attention little if any serious effort has ever been directed to the energy savings possible by the integration of these processes. Evolution not invention predominated the picture.
As each group perceived a problem in its area of expertise, or if operational problems developed after construction, enterprising engineering, inventor, entrepreneur types were brought into the picture. This approach has resulted in the application of inventive genius to a lot of fixes, a lot of complexity, and a large increase in construction, maintenance, and energy costs in WWTPs.
Domestic wastewater, liquefied bio-waste, commercial and industrial liquid waste processes have historically used two distinct classes or systems of bacteria to reduce the biosolids contained therein to more biochemically safe water and solids that can be used for fertilizer and a variety of other products. These two bacterial systems are termed aerobic and anaerobic.
Aerobic processes require the mixing of air or pure oxygen into the liquor being treated so that aerobies (aerobic bacteria) grow, attack, and bio-chemically reduce the solids. Aerobic processes are relatively easy to devise and there are many such systems in use worldwide.
The drive for higher and higher quality effluents has contributed to the expansion and proliferation of aerobic processes. However, there are a number of disadvantages to aerobic processes: they are in general open processes that have odor problems; they tend to require large tanks or ponds that require considerable space; and they consume large quantities of energy in the form of electrical power. Sixty to seventy percent of the energy required in modern domestic wastewater treatment plants is directly attributed to aerobic processes.
Conversely, anaerobic processes can be net energy producers. They operate in closed tanks or vessels devoid of oxygen, at an elevated temperature, are more difficult to control, produce a gas that contains approximately 64% methane (natural gas), 34% carbon dioxide, and 2% hydrogen sulfide. It is the methane component of this raw gas mixture that is valuable for its energy content (nominally 1000 Btu/ft3). Gas production rates are a function of the type and density of the bio-feedstock and general digester efficiency.
The limiting factor that has prevented all wastewater feedstock from being treated anaerobicly is the high ratio of water to bio-solids (volatile solids or VS) contained in the feedstock. Domestic wastewater typically exhibits as little as 0.01% VS. And yet, it is normally difficult to maintain anaerobic action below a minimum threshold of about 3 to 5% VS. Therefore common practice limits anaerobic digestion to that relatively small part of the influent that either settles readily or floats to the top of large primary and secondary sedimentation tanks, thus delegating a very large portion of the influent to aerobic activated sludge processes. The invention described herein completely eliminates this minimum VS requirement so that all biosolid liquor mixtures may be anaerobicly reduced irrespective of their biosolid (VS) concentrations.
The energy produced by anaerobic systems in the form of methane gas is a direct function of the quantity of biomass reduced (VSR) in the process. Therefore, the net positive energy generated is normally severely limited by the water to VS ratio of the digester influent, irrespective of the several chemical-thermal-mechanical factors that determine digester efficiency. And, depending upon the feedstock there has normally been an operating point at which it becomes more efficient to delegate a portion of the treated influent to aerobic processing. This limitation can be overcome to some extent by the addition of external biosolids such as: food, animal, agricultural, grass clippings, tree trimming, cardboard, and other bio-waste products to the anaerobic influent. Therefore, the ability of this invention to control and maintain the desired water to VS ratio in the digester eliminates the necessity for, but not the usefulness of, such considerations.
Anaerobic digesters have been operated in a number of temperature ranges. This invention is applicable to all anaerobic digesters regardless of temperature. Most common digesters operate in the mesophilic range of approximately 35xc2x0 C. or the thermophilic range (55xc2x0 C.). The preferred embodiment and the description of this invention refer to thermophilic operation.
In an anaerobic reactor, retort, or vessel operated in the thermophilic bacterial temperature range of approximately 53 to 58xc2x0 C. (nominally 55xc2x0 C.) there is a certain space above the liquor (hereinafter referred to as the dome, however this reference does not necessarily limit the shape of the vessel) that collects the raw gas produced in the reactor by the anaerobic action. The constituents of this raw gas vary a few percentage points but generally may be expressed as being 60% methane (CH4), 31% carbon dioxide (CO2), 1% hydrogen sulfide (H2S), and 8% water vapor (H2O).
The partial pressures of these gases are a function of the temperature and pressure in the dome. The quantity of methane, carbon dioxide, and hydrogen sulfide available to this mixture is limited by the digester gas production rate. Only the water content of the liquor, the temperature of the vessel, and the pressure in the dome however, limit the quantity of the water vapor available. Since the liquor is generally more than 95% water the quantity of water vapor available may be considered infinite within the confines of this discussion. And, since the temperature of the vessel is set by the anaerobic requirement the surface of the liquor may be considered constant at 55xc2x0 C. However, the total pressure and to some extent the temperature in the gaseous space of the dome above the liquor may be varied widely and in itself will have virtually no effect upon the temperature or operation of the digestion process. Decreasing the pressure will increase the partial pressure of water vapor increasing the ratio of water vapor to gas. It is a major action and purpose of this invention to decrease the absolute dome pressure (creating a vacuum); thus increasing the percentage of water vapor; drawing off this water vapor; to result in the lowering of the water to VS ratio in the vessel, at a rate and to an extent that maximizes methane production and VSR.
At the temperature of 55xc2x0 C. water boils under a vacuum of 12 psi relative to standard conditions. In the range between atmospheric pressure (14.7 psia) and 12 psi vacuum (3 psia) the water vapor available to be drawn off by the process increases in a linear fashion and heat is drawn from the process. At the boiling point however, the rapid boiling of the water impedes further reduction in pressure. At this low pressure the rate of boiling is a function of the heat of vaporization resulting in a sharp rise in the heat drawn from the system. There is a sharp rise in additional heat that must be applied to the vessel influent in order to maintain thermophilic temperature. This invention provides for the condensation and heat exchange of this primarily water vapor or steam-gas mixture flow to the influent feedstock dramatically reducing the requirement for make up heat to the system. The vacuum created in the retort dome is created and controlled by condensing action and the pumping or pulling off by gravity of this condensate gas mixture. The pumping of this predominantly water mixture by gas diffusion liquid pumping or elevated water column condensation are the preferred embodiments of this invention. Embodiments may be performed by special vortex, centrifugal, or other technology. This invention additionally makes use of the facts: that biosolid feedstock at higher operating temperatures demonstrate improved solids separation characteristics; that hot water, vapor, or steam introduced above the liquor level is more readily drawn off into the condenser heat exchange unit; and that heated feedstock introduced to the dome above the scum layer helps to break up this layer, reduces foaming, and makes use of the additional surface area of this rough and uneven surface to enhance vaporization in the dome.
Typically the Raw Gas generated in wastewater, solid waste and/or landfill processes is recycled in order to provide mixing of the liquor in the digestion process. The use of raw gas in mixing is an aid to bacterial growths that break down the bio-solids in the anaerobic digestion process. Raw gas is recycled in the digestion process by one of various methods. The process described herein applies to all gas mixing methods.
Anaerobic digestion is basically a two-phase process. It is the combined action of two forms of bacteria that live together in the same environment and are commonly referred to as the xe2x80x9cacid formersxe2x80x9d and xe2x80x9cmethane fermentersxe2x80x9d. It has been found advantageous to separate or transition these phases to some degree as an aid to keeping a balance between these two bacteria. Acid formers are abundant in raw sewage. Methane fermenters are not nearly so prevalent and require a pH of about 6.6 to 7.6 to produce. A digester is sensitive to too much food, it may easily become too acidic and xe2x80x9cgo sourxe2x80x9d and fail to produce the desired innocuous dewaterable sludge and valuable methane (CH4) if the acid phase is allowed to predominate. The method described herein separates the components of the mixing gas, controls the volumetric gas flow, and stabilizes the above condition over a much broader range of feed conditions by regulating the content of the mixing gas to the various phases.
The carbon dioxide content forms carbonic acid (H2CO3), which when returned to the digester as a component of the mixing gas moves the balance in the direction of the acid formers and can be used to enhance their activity in the acid phase. However the methane component is a valuable constituent both as a gas that may be drawn off the system for energy and as a mixing gas. Many believe that its presence is a further aid to the health of methane fermenters. Natural gas, which contains a high percentage of methane, is sometimes used to restart sour (acidic) digesters. As a mixing gas methane slows the acid forming phase and enhances the fermenting phase. The water vapor and hydrogen sulfide gas that form hydrosulfuric acid a destructively active acid component that makes itself a general nuisance by condensing out in lines, corroding expensive equipment, and the instrumentation used to monitor and control gas flow are entirely removed from the process.
Several methods for separating (scrubbing) methane from the other raw gas constituents have been developed. The application of these processes to wastewater treatment plants has not proven practical and/or economically feasible. The method and process described herein not only provides clean methane gas that is dry cold and dense and does it at atmospheric pressure but also overcomes all of the drawbacks connected with other processes and enhances the operational efficiency of the plant operation.
The Thermophilic Digestion Process is not new. However, like the various gas scrubbing techniques, it has seldom previously been considered an economically viable solution to the treatment of biosolids in a full sized digester. Until the advent of this invention there has never been available an economical heat source capable of maintaining the additional 20xc2x0 C. required for thermophilic operation. Thermophilic digestion is three (plus) times as fast as mesophilic digestion. For instance, this process can reduce the same amount of volatile solids in 10 days that a mesophilic digester will reduce in 32 days. It follows that there is three (plus) times the production of methane gas. By providing the heat required to raise and maintain the operating temperature at the thermophilic range the xe2x80x9cbasicxe2x80x9d digester operating efficiency has increased by a factor of three.
Utilization of the methane energy component of this raw gas has been hampered by the presence of the other by-product components of anaerobic digestion. The most insidiously harmful of these components with respect to repair, maintenance and replacement of equipment in the mixing and/or gas to energy systems (i.e. pumps, blowers, compressors, boiler tubes, cylinders, etc.) is the condensing water vapor and the dilute hydrosulfuric acid (H2SO4+H2O) product of the hydrogen sulfide and water constituents. Operations personnel soon become discouraged by the unreliability, high maintenance costs, and the low time between failures associated with these equipments. So much so in fact that many plants have abandoned the use of raw gas altogether in favor of natural gas (domestic or pipeline), opting to flare-off the raw digester gas and its harmful components to the atmosphere. The invention herein described greatly increases the traditional mean time between failure (mtbf) of all of these equipments.
In addition, the volumetric inefficiency of using a gas that is 40% inert in gas-engines used for pumping or generating systems requires much larger and more expensive engines than the service requirement would otherwise dictate, accompanied by a corresponding reduction in operating efficiency. Additionally passing this carbon dioxide through the combustion process increases the xe2x80x9cgreen housexe2x80x9d effect upon the atmosphere. The invention herein described eliminates all of these drawbacks and limitations providing a positive environmental impact.
Secondary sludge (i.e. sludge from the secondary sedimentation basins and the aerobic treatment processes) tends to be thinner than primary sludge (i.e. sludge from the primary sedimentation basins). In order to handle secondary sludge effectively, whether it is used in direct land application or cycled through the digester, it is customary to thicken this sludge. This requires rather elaborate and expensive apparatus with certain chemicals such as polymers to aid the process. By cycling all sludge through the retort digester the requirement for a separate sludge thickening process is eliminated. Thickening of the sludge in the digester takes place automatically and continually as the water is removed from the vacuum retort digester.
Most recently the United States Environmental Protection Agency in the Code of Federal Regulations 40 Section No. 503 has mandated that in order for sludge to be classified as Class A for unrestricted use or access land application it must be pasteurized to below harmful pathogens levels. Subjecting sludge to thermophilic temperatures for as little as one quarter hour provides this pasteurization. Systems of this type have been in use in Europe for a number of years and are becoming more prevalent in the United States. The equipment involved in such a process is expensive, and only augments digestion process meanwhile adding complexity to the treatment process. The method and process contained herein subjects all sludge to pasteurization, completely eliminating the need for a separate process while providing a positive environmental impact.
When applied to retrofit digesters or installed as a partial treatment process in an existing plant in which the distilled process water is to be added to the plant effluent only partial de-carbonization need take place in the system. The remaining carbonization buffers effluent water. Typically the effluent of wastewater treatment plants tends to run alkaline (i.e., pH in excess of 7.0). In a large number of plants with certain types of industrial influent the pH range becomes excessive. Equipment is being installed in WWTPs today solely for the injection of bottled CO2 gas into the effluent prior to chlorination or other disinfection process. Carbon dioxide provides superior process control by virtue of its self-buffering characteristics. It is being used to replace older systems that use hydrochloric acid, sulfuric acid or acetic acid. The method and process contained herein completely eliminates the need for either of these systems. The carbon dioxide and hydrogen sulfide removed from the raw digester gas is dissolved in the water that is inserted into the secondary effluent stream. In addition, maintaining this buffer reduces the amount of chlorine required for disinfection and reduces the discharge of carbon dioxide to the atmosphere.
It is in general an object of the invention to provide a new and improved anaerobic digestion system and process.
Another object of the invention is to provide an anaerobic system and process of the above character which provide 100% energy independence, zero waste products, zero environmental pollution, potable distilled water production, and a pasteurized fertilizer, sludge or sludge cake product, in a closed bio-thermodynamic systems.
These and other objects are achieved in accordance with the invention by:
(a) Concentrating the liquor in the digester and boiling off water in excess of the amount required for optimizing the digestion process, by creating and controlling a vacuum in the enclosed space above the liquor;
(b) Producing clean, dry, pure methane gas from the raw gas produced in the digester, as disclosed in U.S. Pat. No. 6,291,232, and utilizing that gas to power and control the process and as a saleable pipeline quality gas and/or electrical energy producing product;
(c) Producing pure carbon dioxide gas, by de-carbonating the effluent water, and utilizing that gas to control the digester operation and as a saleable by-product of the process;
(d) Controlling the pH of the digester to increase the digestive action without the use of chemical additives (Steiner 5,630,492), by selectively reintroducing the separated components of the raw gas into the several phases or stages of the digester as an injection additive to the mixing liquor or as the mixing gas itself;
(e) Providing a biologically and chemically pure liquid or potable distilled water effluent by condensing out the water vapor produced in paragraph (a) above; and stripping out all the dissolved gas constituents of that condensate;
(f) Providing a system and process for 100% digestible volatile solids reduction (VSR), that provides extreme flexibility and control of solids retention time (SRT), and insures the production of completely pasteurized biosolids by eliminating the possibility of short-circuiting within the digester from effecting pasteurization;
(g) Providing a vacuum drying system for post digester sludge drying and/or sludge cake production, by extending and utilizing the vacuum created in the digester dome to a drying oven; and
(h) Producing the useful, saleable chemical by-product sodium sulfate (Epson Salts, and an ingredient in detergents, ink and other products), as a first step precipitant during the reactive neutralization of the chilling water supply by the reduction of hydrogen sulfide with sodium hydroxide.
The invention may be used as a standalone liquid waste (wastewater) plant or as a progressive retrofit or addition to an existing plant. This invention is applicable to and improves the cost effectiveness of wastewater treatment plants with flows as low as a few thousand gallons per day to plants with flows of several hundred million gallons per day and with biomass concentrations as low as 0.005% VS (volatile solids) concentrations. The invention is applicable to: all currently known digester systems, including but not limited to single and multi-phase, multi-staged, temperature phased (U.S. Pat. No. 5,525,228), acid phased, mesophilic, thermophilic, suspended growth, up-flow, down-flow, granular, enhanced granular, fluidized bed, attached growth, and the various combinations of the same; including but not limited to those digesters that employ filter media whether fully or partially packed.